Can I use Orb of Regret on my opponent's turn?

No, you cannot use the activated ability of Orb of Regret on your opponent's turn.

In Riftbound, activated abilities (like the one on Orb of Regret) can generally only be used on your own turn when you have priority. Because the ability on Orb of Regret does not have the Reaction keyword, it cannot be activated in response to your opponent's actions or during their turn.